Easy DOD Question

rene26
rene26 Registered Users Posts: 5 ✭
in Off Grid Solar & Battery Systems #1
I've got what I think is an easy one for you here...

Most charts I see showing battery SOC and their related voltages show a 12V battery, at 50% SOC, having a voltage of 12.06V. A battery is considered to be completely discharged when the voltage reaches 10.5V.

Now, my manual for a Magnum 4448PAE inv/chrg tells me to set my low battery cut out--if I want to cycle down to 50%--to 10.0 to 11.4VDC (for 12V systems), or 40.0 to to 45.6VDC (for 48V systems). How in the world can this be? How can setting a low battery cut out at 10V represent a 50% DOD? It is below the commonly accepted value for a 0% SOC battery.

Is it because the batteries, under load, show a lower value than they actually are, and then when the load is removed they snap back to a higher voltage?

Thanks for your replies.
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Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
    Re: Easy DOD Question

    Hi Rene26, welcome to the form.
    Good question :)
    In reality, the low battery cutoff with inverters, is to protect the inverter, not so much the battery. How can that be? Well the lower the battery voltage goes, the greater the current the inverter must handle in order to maintain it's output power in watts. With a heavy load, there comes a point as the battery voltage lowers, when the current that must be drained from the battery in order to maintain the wattage demanded by the load, becomes higher than the inverter can handle without emitting smoke. Thus to save the inverter, it shuts down. Watts = Voltage X Amps.
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  • CATraveler
    CATraveler Solar Expert Posts: 98 ✭✭✭✭
    #3
    Re: Easy DOD Question

    Batteries voltage under load is lowered and does not reflect SOC. It may take several hours of absoutly no load to reflect SOC. The best choices are a hydrometer to measue SP or a battery monitor to measue the actual power added/removed from the batteries.

    An inverter can be a very heavy load that significantly lowers the voltage during operation.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Easy DOD Question

    the guys pretty well nailed it as dod voltages and the inverse of the dod the soc voltages are at rest states of a battery voltage. under load it (the voltage) goes down and during charging it can go down, stay the same, or go up depending on the current being inputted vs the current being outputted (if there is a load on it).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Easy DOD Question

    New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
    leaf wrote: »
    Am trying to upload the charts I am using...

    Attachment not found.Attachment not found.

    attachment.php?attachmentid=3655

    attachment.php?attachmentid=3654

    I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

    Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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